In mechanical watches, escapements have to satisfy several safety criteria. One of the safety devices, the anti-trip system, is designed to prevent the angular extension of the balance beyond a normal angle of rotation. This anti-trip system limits the angle of pivoting of a balance during excessive accelerations, in particular in the event of shocks. This system is essential for certain types of escapement, in particular detent escapements. The anti-trip mechanism must be capable of acting in both directions of pivoting of the balance, i.e. both during extension and contraction of the balance spring.
EP Patent 1801 668 B1 in the name of Montres Breguet SA proposes a system whose structure is characterized in that it includes a pinion mounted on the balance staff. This pinion meshes with a toothed wheel, at least one crossing of which abuts against a fixed stop if the balance is driven beyond its normal angle of rotation. However, this mechanism has an effect on the inertia of the balance and may disturb its oscillations. The gearing comprised therein generates friction which impairs efficiency and may also disturb the regulating system.
EP Patent Application No 1 666 990 A2 in the name of Montres Breguet SA discloses another anti-trip system based on the expansion of the balance spring. A locking arm, fixed to the outer coil of the balance spring, is inserted between a finger integral with the balance and two columns integral with the balance bar. Locking only occurs in the event of excessive expansion of the balance spring beyond an angle exceeding its normal operating angle. However, this mechanism only limits the angle of rotation in one direction of rotation, whereas it is preferable to limit the angle of rotation in both directions of rotation.
A system disclosed in EP Patent No 2450756 A1 in the name of Nivarox-Far SA uses a plate integral with the balance which guides a pin in a spiral-shaped groove. During abnormal oscillations, the pin, integral with a pivoting arm, is stopped and abruptly limits the oscillation. The friction inherent in this mechanism causes interference in the balance throughout its oscillations.
EP Patent No 2 196 867 A1 in the name of Montres Breguet SA discloses a silicon balance spring with a raised coil, which includes an outer coil and an end coil which are connected to each other by a raising device which may include braces used as connecting members or spacers between these two coils. The coils have no contact with other components except the actual balance spring.
U.S. Pat. No. 3,041,819 A in the name of George Ensign discloses a sprung balance with balance spring expansion limiting means, which are formed, on the one hand, by a pin mounted on the balance and extending parallel to the axis thereof, and on the other hand, by a stop block secured to an outer coil of the balance spring.
U.S. Pat. No. 3,696,687 A in the name of Philip Harland discloses a plastic hairspring, which includes a large number of connecting bridges to allow material to flow during moulding, these bridges then being cut to release the coils, without having any other particular function.
EP Patent No 2 434 353 A1 in the name of Montres Breguet SA discloses an anti-trip balance spring wherein notches pertaining to consecutive coils are hooked to each other, both during contraction and extension of the balance spring.